Detecting physical properties of fluids in a vessel is important for a variety of reasons. Many applications, for example, marine and aviation applications, ground based vehicles, vessels and industrial processes require accurate measurements of fuel in a tank to ensure sufficient supplies to reach intended destinations. It is exceptionally important in aviation applications to monitor the fuel levels in multiple tanks to ensure proper balance of levels to impart the least impact on the aerodynamics of an aircraft, which can be significantly affected by changes in the three-dimensional center of gravity of a plane.
An accurate, reliable and safe method of measuring the amount of fluid in a container is essential. Present applications include fuel tanks containing volatile fluids, although the invention described herein can accommodate a wide range of fluids, regardless of their volatility characteristics. Other parameters that must be ascertained with accuracy and consistency are the type of fuel and the contamination content, if any. A further consideration is a need for hardware that meets the electromagnetic interference (EMI), electrostatic discharge (ESD) and interface requirements of a container, such as an aviation fuel tank, in its environment in a safe manner.
Prior radar technology includes methods to scan, lock on and track targets. The basic approach is to transmit a signal that, using radar terms, illuminates targets, performs gating on a receiver to locate targets and, optionally, selects targets to lock onto and track. Analysis of the received signal can then be used to determine the distance (range) of the target and to perform signature recognition to define the type of target and its characteristics. Combining radar technology with transmission line theory solves the problems attendant with sensing fluid levels in containers, particularly those used in the aviation field.
Time Domain Reflectometry (TDR) combines elements of radar technology with digital signal processing. A description of using TDR to detect fluid levels in a vessel is disclosed in the parent application, copending U.S. nonprovisional patent application entitled “Scan Lock and Track Fluid Characterization and Level Sensor Apparatus and Method,” having Ser. No. 12/630,225, filed Dec. 3, 2009.
TDR combines elements of radar technology with digital signal processing. The radar component involves generating a signal, sometimes called an “interrogation pulse,” and transmitting that signal into a vessel, for example, a fuel tank. An interrogation pulse may be, for example, a unit impulse, or a unit step function. The interrogation pulse may be transmitted with a waveguide, for example, a transmission line, a coaxial cable or a coaxial probe. The propagation speed of the interrogation pulse through a material is directly related to the relative permittivity (dielectric constant) of a material. Materials with different dielectric constants will have different propagation speeds. The transit time of the pulse is used to measure the dielectric constant. The propagation speed of the interrogation pulse depends upon the properties of the medium it is traveling through, according to the relationship demonstrated by the equation
                    v        =                  c                      ɛ                                              (                  Eq          .                                          ⁢          1                )            
where ν=velocity of propagation, c=speed of light, and ∈=the dielectric constant. The dielectric constant varies depending on material, and the dielectric constant of many materials can be a strong function of density (and thus of temperature), and is often a strong function of the amount of any additive or contaminant that may be present. Therefore, the velocity of propagation of the traveling pulse is in general changing as it goes from one material to another, and the velocity of propagation in any given medium may moreover vary in correspondence to such factors as additive content and temperature. The effect of temperature on the dielectric constant is especially true of liquids, and the effect of additive content on the dielectric constant is especially true of ethanol additive in hydrocarbon fuel.
Digital signal processing may be used to assist the resolution of the multiple reflected interrogation pulses. Generally, under the Nyquist theory, a waveform must be sampled at least at twice the highest frequency component of the waveform. But due to the combination of the high propagation speeds of the interrogation pulse and the short distances traveled by the interrogation pulses (typically the distance from the top of a fuel tank to the bottom of a fuel tank, and back), the sampling frequency must be extremely high. TDR takes advantage of the fact that the contents of the vessel being monitored change very slowly in relation to the propagation time of the interrogation pulse. Therefore, the reflection of a first interrogation pulse will be, for all practical purposes, indistinguishable from the reflection of a second interrogation pulse transmitted, for example, several nanoseconds later. This obviates the need to sample the received reflected impulse at the Nyquist frequency (twice the frequency of the highest component frequency present in the sampled waveform).
Instead of sampling an entire received waveform at the Nyquist frequency, TDR creates a “time expanded” composite of a sampled waveform by accumulating one or more samples of each reflected impulse. In order to create the composite, the delay between the transmitted pulse and the instant a sample is collected is progressively swept, so that the time difference between successive samples is less than or equal to the period corresponding to the Nyquist frequency. There is no objection to sampling at a higher frequency (for instance, collecting two or more samples for every interrogation pulse). The composite may be created by superimposing samples collected over multiple interrogation pulses. The operation on the sampled signal may then be performed by the signal processing circuitry as if the composite represented a single reflected waveform sampled at or above the Nyquist frequency.
The relatively low sampling frequency reduces the processing load on the signal processing circuitry, and similarly, allows the signal processing to be performed by lower speed and lower cost components. The processing load can be further reduced by only sampling the reflected waveform at selective times. For instance, there may be little interest in analyzing the portion of the reflected waveform corresponding to the reflections generated by impedance transitions that occur before the interrogation pulse enters the vessel, such as the point where the signal is transitioning from the transmission cable to the waveguide. Therefore the sampling time window may be restricted so that the received waveform consists of only reflections from a region of interest, such as the portion of a fuel tank containing fuel.
Where a plurality of stratifying fluids is present within a tank, it may furthermore be desirable to know the height of each stratified fluid layer. For example, where water is mixed with hydrocarbon fuel intentionally, such as when seawater is used as ballast in oil tankers; or unintentionally, such as when water is present in a vehicle fuel tank or such as when groundwater seeps into tanks for fuel pumps at filling stations or due to condensation, it may be desired to know the height of fuel layer(s) as distinct from nonfuel layer(s) for accurate determination of remaining fuel. The detection and measurement of stratified fluid layers is discussed in copending U.S. nonprovisional patent application entitled “System and Method for Optimizing Sweep Delay And Aliasing For Time Domain Reflectometric Measurement of Liquid Height Within A Tank,” having Ser. No. 12/630,305, filed Dec. 3, 2009, which is hereby incorporated herein by reference in its entirety.
Another important measurement function is to ascertain the presence of, and/or amount of, any contaminants in fuel to ensure the safe and proper operation of engines operated with the fuel. Entry of contaminants into an operating engine can lead to severe performance problems and even engine failure. A means to constantly monitor the presence and amount of contaminants, particularly water and ice, is an important component of a fuel measurement system.
Water may be present in an aircraft fuel tank in several forms. Depending upon the form it takes, water can represent different problems to the operation of an aircraft. Water may be dissolved in another fluid, emulsified with an immiscible fluid, or free water may collect and form a water layer. In addition, depending upon the temperature, water may be present as an ice layer or as an ice-fuel mixture, forming a gel like substance. Water may be introduced to a fuel tank in different ways. Water may enter an aircraft fuel tank in the form of water vapor introduced through vents in fuel tanks, or may be introduced to the vehicle in the fuel itself as a solution. Water can accumulate and freeze, clogging fuel tanks and fuel lines. It is standard procedure in some aircraft to routinely check for the presence of water in a fuel tank after a certain time of flight.
A water mixture may exist in fuel in two states: dissolved water (single phase) or emulsified water (two phase). The amount of water present in fuel depends on the fuel grade and the fuel temperature. When water is dissolved it becomes part of the solution based on water molecule bonding, so it is impractical to remove. A small amount of dissolved water is normally found in fuel.
An emulsion is a mixture of two or more immiscible fluids, that is, a mixture of two or more fluids that are unblendable. A first fluid is dispersed in a second fluid, where the second fluid is in a continuous phase. The first fluid is said to be in a dispersed phase. The boundary between the dispersed phase and the continuous phase is called the interface. Emulsions generally appear cloudy or hazy, because the phase interfaces tend to scatter light. Fuel tanks in aircraft are constantly agitated, for example, by pumps, turbulence, and motion of the aircraft. This agitation may not allow emulsified water in fuel to separate and settle. If the temperature drops when, for instance, the aircraft climbs in altitude, the emulsified water may start to freeze, causing the fuel to gel.
Dissolved or emulsified water has traditionally been detected in fuel with chemical test kits. This typically involves taking a sample of fuel, adding water sensitive powder, and looking for a change of color. Then the color of the fuel is compared to a standard color card to determine if water is present.
Free water describes liquid water that is not in a dissolved or emulsified state. Free water generally settles as a stratified layer at the bottom of a tank, below the fuel. For general aviation craft, that is, small, generally propeller driven aircraft, the simplest method for detecting free water in fuel is a manual check before takeoff. A device is inserted into a valve at the bottom of the fuel tank that draws some fuel from the bottom of the tank. A fuel sample is drawn from the bottom of the tank because water is denser than fuel and therefore settles at the bottom of the tank. Once the sample is drawn, the pilot visually inspects the fuel sample for water. The water appears as clear bubbles or droplets. The fuel is usually dyed a color, which makes water bubbles visually distinguishable from the fuel. If water is detected, more fuel is drained from the bottom of the tank, and then another sample is again visually inspected. This process is repeated until the sample appears free from water. However, this process does not detect water that may be emulsified and thereafter settles out during flight, or water introduced to the fuel during flight.
Larger aircraft may have built-in water detecting systems. However, existing water detecting systems may not be accurate in many scenarios and may not be able to determine the amount of fluid in stratified layers, and further not be able to track or detect changes in fluid characteristics. Also, most current water detecting devices may not be able to detect or differentiate water from ice. While there are existing methods of water detection in aircraft, it is important to have redundant backup systems. Therefore, it is desirable for the fuel level detecting system to also detect free water or ice, as well as contamination in fuel. However, a capacitance probe, which has traditionally been used to detect fluid levels in tanks, cannot measure the impedance across two or more stratified layers of a tank. If the capacitance probe crosses a layer boundary, the resulting reported impedance will be a value somewhere between the impedance of each layer. So a capacitance probe is particularly ill suited for simultaneously detecting the fuel level and for detecting free water in a tank. For this reason, capacitance probes are often deployed so that they do not extend into the bottom inch or two of a fuel tank, where water is most likely to accumulate. Therefore, there is a need for a fuel level detector to monitor fuel quality in real time and to detect water and other contaminants.